<template>
    <ul class="list">
        <li class="item" v-for="item of letters" 
         v-bind:key="item"
         v-bind:ref="item"
         v-on:touchstart.prevent="handleTouchStart"
         v-on:touchmove="handleTouchMove"
         v-on:touchend="handleTouchEnd"
         v-on:click="handleLetterClick" 
         >{{item}}</li>
    </ul>
</template>
 
 <script>
  export default {
    name: 'CityAlphabet',
    props: {
      cities: Object
    },
    computed: {
      letters() {
        const letters = []
        for (let i in this.cities) {
          letters.push(i)
        }
        return letters
      }
      
    },
    data () {
      return {
        touchStatus: false,
        startY: 0,
        timer: null,
      }
    },
    updated () {
      this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick (e) {
          this.$emit('change',e.target.innerText)
        },
        handleTouchStart () {
          this.touchStatus = true
        },
        handleTouchMove (e) {
          if (this.touchStatus) {
            if (this.timer) {
              clearTimeout(this.timer)
            }
            this.timer = setTimeout(() => {
              const touchY = e.touches[0].clientY - 79
              const index = Math.floor((touchY - this.startY) / 20)   
              if (index >= 0 && index < this.letters.length) {
                this.$emit('change',this.letters[index])
              }  
            }, 16)      
          }
        },
        handleTouchEnd () {
            this.touchStatus = false
        },
    }
  }
 </script>

<style lang="stylus" scoped>
  @import '~__STYLES__/varibles.styl'
  .list
    display: flex
    flex-direction: column
    justify-content: center
    position: absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      line-height: .4rem
      text-align: center
      color: $bgColor
</style>